This invention relates to a method of epitaxially growing II-VI semiconducting compounds such as ZnS, ZnSe and ZnTe as well as solid solutions and superlattice crystals comprised thereof.
Having a wide gap of the direct transition type, II-VI semiconducting compounds such as ZnS, ZnSe and ZnTe can emit ultraviolet, blue and green light with high efficiency. Since there is yet to be developed a technology of growing crystals capable of controlling both the p and n conductive types, however, the fields of their application as industrially acceptable semiconducting elements are extremely limited. Among these compounds, only ZnS is considered practically acceptable and used mainly as a phosphor.
Recently, there have been efforts to use the new technologies of growing thin film single crystals such as the organic metal chemical vapor deposition method and the molecular beam epitaxy method to realize an injection-type high-efficiency light emitting element in the range between short-wavelength visible and ultraviolet which cannot be realized with other compounds. The new growth technologies such as the organic metal chemical vapor deposition method and the molecular beam epitaxy method are now nearly established as methods by which high-purity crystals of III-V semiconducting compounds can be controllably obtained. It is therefore desirable to establish similar technologies for II-VI semiconducting compounds. If the conventional metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy method is used to epitaxially grow a II-VI semiconducting compound, however, a III-V semiconducting compound with a similar lattice constant, etc. is used as the substrate for the growth but there remain many problems which tend to counteract the advantages of these new growth technologies. In the heteroepitaxial growth using a substrate of GaAs or the like, for example, defects caused by a misfitting occur inevitably because the lattice constants do not match. In order to match the lattice constants, attempts may be made to add a new element to make a mixed crystal substrate or alternatively to grow a mixed crystal but it is extremely difficult to achieve a precise control with good reproducibility. Accordingly, the problem of defects caused by misfittings is extremely difficult to solve. Moreover, since the substrate and the crystal to be grown are of different types, their coefficients of thermal expansion are different and a tensile force or a compressive force inevitably acts on the crystal being grown. Defects are therefore induced as the temperature changes after the growth, for example, when the temperature is reduced or during a subsequent heat treatment. Furthermore, III-V semiconducting compounds and semiconducting elements of Group IV act materially as impurities in a II-VI semiconducting compound. Since they inevitably become mixed into the crystal during its growth, they tend to reduce the purity of the grown crystal. It now goes without saying that these problems are extremely serious obstacles to obtaining high-quality crystals with a low defect density which are necessary for the control of physical characteristics (p and n conductive-type control) of II-VI semiconducting compounds.